Forming of materials

ABSTRACT

A process for producing an extrusion of small cross section from a workpiece in which the workpiece is subjected to a bulk compressive stress in a container so that the end of the workpiece is forced into a reducing die at the end of the container. The material of the workpiece in the reducing die is subjected to an additional localised compressive stress by a tool member having a working face which is applied to the material of the workpiece in the reducing die. Under the combined compressive stresses the material of the workpiece is formed through a die orifice. In one arrangement a rotary tool member is employed which is moved in a circular path through the workpiece material in the reducing die. In another arrangement the tool member is fixed against rotation and the workpiece is rotated.

United States Patent 1191 Green July 23, 1974 FORMING OF MATERIALS 3,459,021 8/1969 Fuchs, Jr. 72 60 3,488,416 l/1970 Rothschild 425/376 [75] Inventor Derek Greg, Lytham Annes, 3,553,996 1/1971 Sabroffet a1. I

England 3,685,147 8/1972 Nevin et a1. 425/131 [73] Assignee: United Kingdom Atomic Energy Authority, London, England Primary Examiner-Richard .1. Herbst [22] Filed May 7 1973 Attorney, Agent, or Firm-Larson, Taylor & Hinds 21 A 1. No.: 357 S59 1 pp 57 ABSTRACT Related US. Application Data A f d f n [63] Continuation-impart of Ser. No. 880,127, Nov. 26, Process or pro ucing exniuslon 0 sm Cross 1969 abandoned sect1on from a workplece 1n wh1ch the workp1ece 1s subjected to a bulk compressive stress in a container so that the end of the workpiece is forced into a re- [3-0] Forelgn Apphcauoli Pnomy D ducing die at the end of the container. The material of M81. 16, 1973 Great Brltam 12847/73 the workpiece in the reducing is Subjected to an additional localised compressive stress by a tool mem- [22] US. Cl]. 72/62, 722232 her having a working face which is applied to the C .f B 1: 2 l 8 terial of workpiece in the reducing Under the [5 1 held 0 9 3 22: 1; combined compressive stresses the material of the l 5/ workpiece is formed through a die orifice. In one'arrangement a rotary tool member is employed which is [56] References cued moved in a circular path through the workpiece mate- UNITED STATES PATENTS rial in the reducing die. In another arrangement the 2,026,979 1/1936 Jones 72/256 tool member is fixed against rotation and the work- 3,306,089 2/1967 Brayman 72/60 iece is rotated. 3,328,998 7/1967 Sabroff et a1. 72/60 p 3,415,088 12/1968 Alexander et al. 72/60 14 Claims, 24 Drawing Figures E i i /7 l7 9 W 4 a I 6 1% i 1 J,

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SHEET U2 HF 15 FATENTED L sum '1sur15 1 FORMING OF MATERIALS This application is a continuation-in-part of parent application Ser. No. 880,127, filed Nov. 26, 2969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the forming of materials and in particular relates to the forming of a product of reduced cross section from a workpiece by an extrusion process.

In extrusion a workpiece is subjected to pressure in a container so that the workpiece is extruded from the container through an orifice defining the product cross section. Pressure may be applied on the workpiece mechanically, as in conventional extrusion, by a ram acting on the workpiece in the container. Alternatively, as in hydrostatic extrusion liquid may be pressurised about the workpiece in the container to effect extrusion of the workpiece.

One feature which is a practical limitation in carrying out such an extrusion process is that the pressure required to carry out extrusion is dependent on the extrusion ratio, the extrusion ratio being defined as the cross sectional area of the workpiece relative to the cross sectional area of the extruded product. workpiece which Even in the case of soft materials very high extrusion ratios (for example in theregion of 500 I) can only be achieved by the application of extremely high pressures (for example 150-200 tons per square inch) on the orkpiece in the container. The manufacture of containers whicb can withstand such high pressures is difficultand costly.

It is one of the objects of the present invention to provide an apparatus capable of producing extruded products at such high extrusion ratios with the application of only moderate pressure to the workpiece in the extrusion container.

SUMMARY OF THE INVENTION According to the present invention an apparatus for producing from a workpiece a product of reduced cross section comprising means for applying a bulk compressive stress to the whole of the workpiece, a tool having a working face of smaller cross-section than the workpiece and mounted for application to a localised region of the workpiece and means for effecting relative rotary movement between the tool and the workpiece whereby the material of the workpiece is subjected to an additional compressive stress at said localised region thereof and is formed through an orifice defining the product cross-section under the combined influence of said bulk compressive stress and said additional compressive stress.

The additional compressive stress may be produced in the localised region of the workpiece by rotating the tool in a circular path with the working face thereof in pressure contact with the material of the workpiece.

In an alternative form of apparatus for carrying out the invention the tool is fixed against rotation, and means are provided for rotating the workpiece with the localised region thereof at all times in pressure engagement with the fixed tool.

In both the above forms of apparatus the container may have a bore tapered at one end to a reduced cross section, the pressure applied to the workpiece in the container also acting to force the end of the workpiece into the tapered end of the bore of the container and the tool being applied to operate on the material of the workpiece in the tapered end of the bore of the con tainer.

DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a longitudinal sectional elevation of one embodiment of the invention, the area bounded by the chain dotted circle 11 being along the line I I in FIG. 2.

FIG. 2 is a partial plan view of the arrangement shown in FIG. 1

FIG. 3 is a detail of FIG. 1 isometric form.

FIG. 4 is a longitudinal sectional elevation of a second embodiment of the invention.

FIG. 5 is a detail of FIG. 4 in isometric form.

FIG. 6 is a longitudinal sectional elevation of a third embodiment of the invention.

FIG. 7 is a longitudinal sectional elevation of a fourth embodiment of the invention.- FIG. 8 is a detail of FIG. 7 on a larger scale.

FIG. 9 is a detail showing a modified form of the arrangement shown in FIG. 7.

FIG. 10 is a longitudinal sectional elevation of a fifth embodiment of the invention.

FIG. 11 is a detail showing a modified form of the arrangement shown in FIG. 10.

FIG. 12 is 'a longitudinal sectional elevation of a sixth embodiment of the invention.

FIG. 13 is a longitudinal sectional elevation of a seventh embodiment of the invention.

FIG. 14 is a longitudinal sectional elevation of an eighth embodiment of the invention.

FIG. 15 is a detail, in isometric form of the arrangement shown in FIG. 14.

FIG. 16 is a longitudinal sectional elevation of a ninth embodiment of the invention.

FIG. 17 is a longitudinal sectional elevation of a tenth embodiment of the invention.

FIG. 18 shows an eleventh embodiment of the invention in isometric form.

FIG. 19 is a detail, in isometric form, showing a modification of the arrangement of FIG. 18.

FIG. 20, is a detail, in isometric form, showing another modification of the arrangement of FIG. 18.

FIG. 21 is a detail, in isometric form, showing a further modification of the arrangement of FIG. 18.

FIG. 22 is a cross section along the line XXII XXII in FIG. 21.

FIG. 23 is a cross sectional detail showing a fourth modification of the arrangement of FIG. 18.

FIG. 24 is a diagrammatic arrangement of yet another embodiment of the invention.

In connection with FIGS. 1017 and the corresponding descriptions in the ensuing specification, it is to be noted that, although the subject matter constitutes part of my generic invention, no claims are directed thereto in the instant application, the subject matter thereof in the instant application, the subject matter thereof being covered in my copending application filed concurrently herewith as a division of patent application Ser. No. 880,127, of which the instant application is a continuation-in-part.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGS. 1 and 2 of the drawings there is shown a pressure container 1, having a bore 2. The bore 2 leads to a narrower outlet opening 3 through a cured section 4 at the end of the bore 2. Alternatively the section 4 may be of straight conical taper or instead of being a concave curvature as shown in the drawing may be of convex curvature somewhat as the shape of a trumpet bell. A tubular shaft 5 projects into the opening 3 from outside the container 1. The shaft 5 is supported in the opening 3 by a heavy duty thrust bearing carried by a hydraulic ram (not shown) so that the shaft 5 can be moved into and out of the opening 3. The shaft 5 is arranged to be driven by an electric motor. A die block 6 is mounted on the upper end of the shaft 5. The die block 6 has a transverse key 7 which fits in slots 8 in the upper end of the shaft 5. As shown in FIG. 3 a die support member 9 is formed projecting from the upper face of the die block 6. The die support member 9 has an inclined flat face 10. A housing 11 in the die support member is fitted with a die insert 12. The housing 11 has a conical lead in 13 from the face of the die support member 9 to the mouth of the die insert 12. The die insert 12 is connected with the bore of the shaft 5 by a passageway 14.

In use of the arrangement shown in the drawings a billet 15 (shown in chain dotted outline in FIG. 1) is held in the container 1. Hydraulic liquid in the interspace 16 surrounding the billet 15 is pressurised to apply an overall compressive stress on the billet 15 so that the end of the billet is forced into the curved section 4 at the end of the bore 2 of the container 1. The shaft 5 is rotated to drive the die block 6. The material of the billet 15 forward ofthe face 10 of the die support member 9 is subjected to an additional localised compressive stress system arising from the mechanical loading applied by the face 10 of the die support member 9 on the billet material, as the die block 6 is rotated. The material of the billet traversed by the die support member during each rotation of the die block 6 is extruded through the die insert 12. Extrusion of the billet material is under the additive effect of the overall compressive stress applied in the billet by pressurisation of the hydraulic liquid about the billet and the localised additional compressive stress which is set up forward of the face 10 of the die support member 9. The extruded wire product passes from the die insert 12 through the passageway 14 and out through the bore of the tubular shaft 5. Extrusion is continuous whilst the shaft 5 is rotated, the billet 15 being fed continuously downwards by the pressure of the hydraulic liquid into the region of the die block 6.

To prevent rotation of the billet 15 in the container 1, the curved section 4 of the container bore 2 may be formed with grooves 17 as shown in FIG. 1, the material of the billet 15 being forced into engagement with the grooves 17.

As disclosed in our copending British Application No 30277/64 cognate with 28823/65 a direct mechanical loading may also be applied on the billet 15 by a ram entered into the rear end of the bore 2 of the container 1. The mechanical loading applied by the ram supplements the axial forces feeding the material of the billet into the region of the die block 6 and also contributes to the stress system giving rise to extrusion of the material of the billet through the die insert 12.

Particularly in the case of soft materials the overall compressive stress may be set up in the billet by direct mechanical loading of the billet for example by a ram as in conventional mechanical extrusion.

By way of example an arrangement for extruding a 2 /2 inch diameter lead billet has the following parameters:

lntemal diameter of die insert 12 0.1 inches Ratio of area of face 10 of the die support member to the area of the die insert orifice 4 1 Pressure in hydraulic liquid 4 tons/square in Ratio of area of container bore 2 area to die block 6 7 l The reduction of a 2 /2 inches diameter lead billet to a wire of 0.1 inch diameter entails a reduction ratio of 625111. At the pressure used ie 4 tons per square inch ratio which can be achieved. From another point of view in order to achieve a reduction ratio of 625 l in a lead billet by simple hydrostatic extrusion a pressure of 60 tons per square inch would be required. This represents a reduction in pressure in the region of a factor of 10.

As a further example an arrangement capable of handling a 4 inch diameter copper billet would have the following parameters:

Diameter of die insert 12 0.125 inches Ratio of area of face 10 of die support member to area of die insert orifice 9:1 Pressure in hydraulic liquid 50 tons per square in. Power of drive motor for shaft 5 25 30 HP Ratio of bore 2 diameter to die block 6 diameter 4:1 Rate of revolution of shaft 5 200 RPM In an arrangement having the above parameters a 4 inch diameter billet is reduced to a wire of 0.125 inches diameter. If this were carried out by a simple hydrostatic extrusion process the extrusion ratio entailed would be approximately 1000:l and in the case of a copper billet a prohibitively high liquid pressure of 165 tons per square inch would be required for simple hydrostatic extrusion.

Although the invention has its main application to the extrusion of harder metals such as copper it may also be used for the extrusion of softer metals such as aluminium. Although a soft metal such as aluminium can be directly reduced to wire by simple hydrostatic extrusion at practical extrusion pressures use of the invention enables a considerable reduction in pressure with saving in cost of pressure vessels and pressure generating equipment which are expensive items.

In FIGS. 4 and 5 of the drawings there is shown a chamber 21 having a bore 22. A reducing die 23 is screw fitted in the end of the bore 22.

The did 23'is sealed in the bore 22 by a copper mitre ring 24 and a rubber O ring 25. A sleeve shaped rotary die block 26 is fitted on a stationary stem 27. The end 28 of the sleeve shaped die block 26 is reduced to fit in the parallel outlet 29 of the reducing die 23. A die member 30 is formed projecting from the annular end face 31 of the die block 26. A die orifice 32 in the die and the sleeve shaped die block 26 is rotatably supported on the stationary stem 27 by a heavy duty bearing (not shown).

In use of the arrangement described above a billet 36 is subjected to the pressure of hydraulic liquid 37 surrounding the billet 36 in the bore 22 of chamber 21. The pressure of the liquid 37 subjects the billet to an overall compressive stress system and also loads the billet 36 longitudinally into the reducing die 23 over the pointed end 34 of the stationary stem 27. The sleeve shaped die block 26 is driven on the stationary stem 27 thus driving the die member 30 through the billet material at the mouth of the reducing die 23. The material of the billet forward of the face of the die member 30 is subjected to an additional localised compressive stress system arising from the mechanical loading applied on the billet material in the reducing die 23 by the face of the die member 30. The material of the billet traversed by the die member 30 is extruded through the die orifice 32. Extrusion of the billet material is under the additive effect of the overall compressive stress applied in the billet by the pressure of the hydraulic liquid and the localised additional compressive stress which is set up in the billet material at the mouth of the reducing die 23 forward of the face of the die member 30.

The wire product extruded through the orifice 32 passes through the passageway 33 in the die block 26 and is coiled on a spool concentric with the stationary stem 27. Under the pressure of the hydraulic liquid 37 the billet 36 is continually fed into the reducing die 23 to replace the billet material which is extruded through the orifice 32 in the die member 30. The pointed end 34 of the stationary steam 27 acts as a guide for feeding of billet material into the region of the annular end face 31 of the sleeve shaped die block 36. The engagement of the flats 35 on the pointed end 34 of the stationary stem 27 with the end of the billet 36 assists in preventing the billet 36 rotating with rotation'of the sleeve shaped die block 26.

The sleeve shaped die block 26 has to be driven under a load sufficient to provide the additional compressive stress in the billet material required to achieve extrusion. In addition part of the driving load applied to the sleeve shaped die block 26 is used in overcoming the friction between the annular end face 31 of the die block 26 and the billet material. As the die block 26 is in frictionalv contact with the billet material only over the relatively small area of its annular end face 31 only a minor proportion of the driving load applied to the die block 26 is used in overcoming friction.

This is to be compared with the arrangement of FIG. 1 in which the full end face of the die block 6 is in contact with the billet. In the arrangement of FIG. 1 there is a greater loss of power as more redundant work has to be done in overcoming the friction between the full end face of the die block 6 and'the billet.

During each rotation of the die block 6 the die member 30 removes a semi toroidal section of the billet material. Depending on the mean diameter (D) of the pitch circle of the rotating die member 30, the pressure (P) in the billet material ahead of the rotating die member 30 and the shear strength (6s) of the billet a maximum area (a) can be defined for the end face of the die member 30 above which the semi toroidal section will shear from the billet instead of extruding. In the case of a die member 30 having an area (a) less than this maximum the semi toroid is clamped ie it is of a circumferential length giving a surface area which will not shear.

FIG. 6 shows an alternative arrangement to that shown in FIGS. 4 and 5. In FIG. 6 there is shown a chamber 41 having a bore 42. A reducing die 43 is screw fitted in the end of the bore 42. The die 43 is sealed in the bore 42 by a copper mitre ring 44 and a rubber O-ring 45. A tubular rotary die block 46 has an end part 47 reduced to fit in the parallel outlet 48 of the reducing die 43. A die member 49 is formed projecting from the annular end face 50 of the die block 46. A die orifice 51 in the die member 49 connects with a passageway 52 leading through the die block 46. The other end of the bore 42 of the chamber 41 is closed by a screwed plug 53 which is sealed in the bore 42 by a copper mitre ring 54 and a rubber O-ring 55. A mandrel 56 integral with the screwed plug 53 extends coaxially through the bore 42 of the chamber 41. The lower end of the mandrel 56 extends into the bore 57 of the die block 46. A passageway 58 leads radially through the wall of the chamber 41 into the bore 42. The rotary die block 46 is supported by a heavy duty bearing (not shown).

In use of the arrangement shown in FIG. 6 a tubular billet 59 is fitted on the mandrel 56 in the bore 42 of the chamber 41. Hydraulic liquid 60 surrounding the billet 59 in the chamber 41 is pressurised through the V radial passageway 58 in the wall of the chamber 41.

The pressure of the liquid subjects the billet 59 to an overall compressive stress system and also loads the billet 59 longitudinally into the reducing die 43. The rotary die block 46 is driven to drive the die member 49 through the billet material at the mouth of the reducing die 43.

The billet material forward of the face of the die member 49 is subjected to an additional localised compressive stress system arising from the mechanical loading applied by the face of the die member 49 on the billet material as the die block 46 is rotated. The material of the billet at the mouth of the reducing die 43 is extruded through the die orifice 51 in the die member 49 under the additive effect of the overall stress applied in the billet material by the pressure of the hydraulic liquid 60 and the localised compressive stress set up in the billet material at the mouth of the reducing die 43 by the loading of the die member 49. The wire product extruded through the die orifice 51 passes through the passageway 52 in the die block 46 and is coiled on an external take up spool. In the arrangement of FIG. 6, as in the arrangement of FIGS. 4 and 5 power losses due to friction between the annular end face 50 of the die block 46 and the billet are reduced as compared with the arrangement of FIG. 1 to 3 in which the die block 6 has a full circular end face in contact with the billet.

The arrangement shown in FIG. 7 of the drawings comprises a pressure container 61 having a longitudinal bore 62. The container 61 is mounted vertically on a base plate 63 by a flange 64 which is screwed onto a boss 65 at the base of the container 61. The flange 64 is secured to the base plate 63 by threaded studs 66. A reducing die 67 is fitted in the upper end of the container bore 62. A circumferential groove 68 around the outside of the reducing die 67 contains an O-ring 69 which seals the die 67 in the container bore 62. The die 67 seats on a base ring 70 which is screwed into the threaded upper end 71 of the container bore 62 up to the limit of an external flange 72 on the base ring 70. A carrier plate 73 rotatably mounted on the upper end of the container 71 by a thrust bearing 74 is fitted with a holder 75 for arotary die block 76. As shown in FIG. 8 the reducing die 67 has a parallel outlet 77 and the rotary die block 76 has an end section 78 of reduced diameter fitting in the parallel outlet 77 of the reducing die 67.

The carrier plate 73 is circular and has an outer rim 79 housing the outer race 80 of the thrust bearing 74. The inner race 81 of the thrust bearing 74 is fitted in a circumferential step 82 around the upper end of the container 61. The holder 75 for the rotary die block 76 is cylindrical with a central drilling 83 and is externally threaded-to screw into a central aperture 84 in the carrier plate 73. The rotary die block 76 is fitted in a counterbore 85 at the lower end of the central drilling 83 in the holder75. The rotary die block 76 has longitudinal splines 86 engaging in keyways 87 in the counterbore 85 of the holder 75. The rotary die block 76 has a blind ended bore 88 corresponding to the central drilling 83 in the holder 75. As also shown in FIG. 8 an oblique drilling 89 in the lower end face 90 of the rotary die block 76 houses an extrusion die 91 which partially projects from the lower end face 90 of the rotary die block 76. A smaller diameter extension 92 of the drilling 89 connects with the bore 88 of the rotary die block 76.

Connection of the bore 62 of the container 61 with a pipe 93 for carrying liquid under high pressure is provided by passageway 94 in the container 61 leading from the boss 65 to the bore 62. The pipe 93 is connected with the passageway 94 at the boss 65 by a union nut 96.

Disc shaped weights 97 are stacked on the carrier plate 73 to load the rotary die block 76. The lowermost weight seats in a step 98 around the upper face of the carrier plate 73.

FIG. 7 of the drawings also shows an arrangement for supporting a billet 99 in the bore 62 of the container 61. This billet support arrangement comprises a blind ended nylon sleeve 100 supported bya coil spring 101 which is mounted on a flanged boss 102 at the bottom end of the bore 62 in the container 61.

FIG. 9 of the drawings shows a modification of the arrangement of FIG. 7. In the arrangement of FIG. 9 the rotary die block 76 has a through drilling 103 fitting about a stationary stem 104 which is mounted from a main frame member of the equipment (not shown). The drilling 103 of the rotary die block 76 has a counterbore 105 at its upper end corresponding to the central drilling 83 in the holder 75. A passageway 106 leads from the extrusion die 91 to the counterbore 105 in the rotary die block 76.-The lower end 107 of the stationary stem 104 is pointed and projects below the lower end face 90 of the rotary die block 76.

In use of the arrangement shown in FIG. 7 of the drawings liquid 108 surrounding the billet 99 in the bore 62 of the chamber 61 is pressurised to subject the billet 99 to compression. Under the action of the pressurized liquid 48 the billet 69 is subjected to an upward thrust so that the nose of the billet 99 is forced into the reducing die 67. The weights 97 load the carrier plate 73 to hold the rotary die block 76 against the upward thrust of the billet 99. Sufficient of the weights 97 are used so as to slighly overload the carrier plate 73, the majority of the weight acting to resist the upward thrust rim 79 of the carrier plate 73. Rotation of the die block 76 drives the die member 91 in a circular path through the material at the reduced end face of the billet 99. The billet material in'the path of the die member 91 is extruded through the die member 91 and the extruded product emerges from the die member 91 through the extension 92 of drilling 89 and is removed through the bore 88 of the rotary die member 16 and the central drilling 83 in the holder 75.

As the die block 76 is rotated the billet 99 is fed continually upwards into the reducing die 67 to replace the material extruded on each rotation of the die block 76.

The arrangement of FIG. 9 operates in a similar manner except that the rotary die block 76 rotates on the stationary stem 104, the lower pointed end 107 of which penetrates the end face of the billet 99. The extruded product passes from the die member 91 through the longitudinal passageway 106 in the rotary die member 76 and is removed through the counterbore of the rotary die member 76 and the central drilling 83 in the holder 75. In the arrangement of FIG. 8 the sliding friction between the annular end face of the rotary die member 76 and the end face of the billet 99 is less than in the arrangement of FIG. 7 wherein the whole circular end face of the rotary die member 76 is in sliding frictional contact with the end face of the billet 99. Therefore in the arrangement of FIG. 8 the work required to overcome friction between the end face of the rotary die member 76 and the end face of the billet, which requires the use of additional power in driving the rotary die member 76, is less than in the arrangement of FIG. 7. Also in the arrangement of FIG. 8 the lower pointed end 107 of the stationary stem 104 feeds the billet material into the path of the die member 91 and static friction between the lower pointed end 107 of the stationary stem 104 and the end face of the billet 99 assists in preventing rotation of the billet 99 in the chamber 61.

In FIG. 10 of the drawings there is shown a chamber 111 having a bore 112. A reducing die 113 is formed at one end of the bore 112 of chamber 111. A plunger 114 isentered into the other end of the bore 112 of chamber 111. The plunger 114 is sealed in the bore 112 by a copper mitre ring 115 and and a rubber O-ring 116. A reciprocable plunger 117 is mounted at the mouth of the reducing die 113 in axial alignment with the bore 112 of the chamber 111. The plunger 117 has a bore 118 which is restricted at its end to form a die orifice 119.

In operation of the apparatus shown in FIG. 10 liquid 120 enveloping a billet 121 in the chamber 111 is pressurised by loading the plunger 114. The liquid 120 is held at a constant pressure sufficient to cause extrusion of the end of the billet 111- into the reducing die 113 up to the end face of the plunger 117. However the pressure in the liquid 120 is insufficient to cause extrusion of the billet 111 through the die orifice 119 in the plunger 117. The plunger 117 is loaded against the reduced end face of the billet 121 at the mouth of the reducing die 113 so that the material of the billet 121 is extruded through the die orifice 119 in the plunger 117 

1. Apparatus for producing from a workpiece a product of reduced cross-section comprising a workpiece, an orifice defining the reduced product cross-section, means for applying a bulk compressive stress to the whole of the workpiece to influence the workpiece to extrude through said orifice, a tool having a working face of smaller cross-section than the workpiece and mounted for application to a localised region of the bulkstressed workpiece adjacent said orifice, and means for effecting relative rotary movement between the tool and the workpiece to apply an additional compressive stress at said localised region to influence the localised region to extrude through said orifice, whereby the material of the workpiece is subjected to an additional compressive stress at said localised region thereof and is formed through the orifice defining the product crosssection under the combined influence of said bulk compressive stress and said additional compressive stress.
 2. Apparatus according to claim 1 including means for rotating the tool in a circular path with the working face thereof in pressure contact with the material of the workpiece.
 3. Apparatus according to claim 1 comprising a container, the bore of the container having a tapered end reducing to a smaller cross-section, means for applying pressure to the workpiece in the container so as to set up a bulk compressive stress in the whole of the workpiece and so as to force the corresponding end of the workpiece into the tapered end of the bore of the container, the tool comprising a rotary member with an end face having a projecting tool member, the tool member having a material working face, means for rotating the rotary member with the end face thereof in contact with the forward end of the workpiece in the tapered end of the bore of the container so that the tool member is moved in a circular path with its material working face in pressure contact with the material of the workpiece.
 4. Apparatus according to claim 3 wherein an orifice defining the product cross-section is formed in the material working face of the tool member.
 5. Apparatus according to claim 3 wherein an orifice defining the product cross-section is formed in the face of the rotary member forward of the material working face of the projecting tool member, which is of solid form.
 6. Apparatus according to claim 3 wherein the rotary member has a plurality of projecting tool members each having an associated orifice defining the product cross-section.
 7. Apparatus according to claim 5 wherein a plurality of orifices defining the product cross-section are formed in the face of the rotary member forward of the material working face of the projecting tool member.
 8. Apparatus according to claim 3 wherein means are provided for the feeding of lubricant to the face of the rotary member behind the projecting tool member.
 9. Apparatus according to claim 3 wherein the rotary member is sleeve shaped and is rotatably mOunted on a stationary stem located within the sleeve, the tool member being formed projecting from the annular end face of the sleeve shaped rotary member, the rotary member being rotated with said annular end face and the corresponding end of the stationary stem in contact with the workpiece.
 10. Apparatus according to claim 9 wherein the end of the stationary stem is pointed and projects from said sleeve whereby the pressure applied to the workpiece in the container causes feed of the material of the workpiece over the pointed end of the stationary stem into the region of the annular end face of the sleeve shaped rotary member.
 11. Apparatus according to claim 3 wherein a mandrel is supported coaxially in the container to adapt the apparatus for handling of a tubular workpiece which fits about the mandrel in the container, the pressure applied to the workpiece in the container causing feed of the material of the workpiece into the tapered end of the bore of the container about the mandrel, the end face of the rotary member having held in contact with the annular end face of the billet in the forward part of the tapered end of the bore of the container so that the tool member projecting from the end face of the rotary member is moved in a circular path to traverse said annular end face of the workpiece.
 12. Apparatus according to claim 1 in which the tool is fixed against rotation, means for rotating the workpiece with the localised region thereof at all times in pressure engagement with the fixed tool.
 13. Apparatus according to claim 12 in which the means for rotating the workpiece comprises a rotatable mandrel supporting the workpiece at the localised region thereof and means for keying the mandrel to the end of the workpiece.
 14. Extrusion apparatus for producing from a workpiece a product of reduced cross-section comprising an extrusion orifice defining the product cross-section, means for locating a workpiece adjacent the inlet of the extrusion orifice, means for applying a bulk compressive stress to the whole of the workpiece to influence the workpiece to extrude through said orifice, means for applying an additional localised compressive stress at a localised region of the workpiece adjacent the inlet end of said orifice to influence the localised region to extrude through said orifice, said last mentioned means comprising a tool having a working face of smaller cross-section than the workpiece and mounted for application to said localised region adjacent said orifice, and means for effecting relative rotary movement between the tool and the workpiece to effect application of said additional localised compressive stress, said tool working face being located radially outwardly from the rotational center of said rotary movement and oriented so as to oppose said relative rotary movement and thus compressively stress said localised region of the workpiece adjacent said orifice, whereby the material of the workpiece is subjected to an additional compressive stress at said localised region thereof and is formed through the orifice defining the product cross-section under the combined influence of said bulk compressive stress and said additional compressive stress. 